1. Field of the Invention
The invention relates in general to a photolithography and etching process of a semiconductor fabrication process. More particularly, this invention relates to a method of forming an opening in a wafer layer.
2. Description of the Related Art
As the dimension of a semiconductor device is further being scaled down, techniques such as phase shift mask (PSM) or optical proximity correction (OPC) mask is used during the exposure process to improve the profile of a photoresist pattern. However, when the pattern dimension is smaller than a half of the wavelength of the light source, the diffraction effect becomes significant. Especially in the case of a contact hole/via hole type of photoresist pattern, the profile of the pattern cannot be effectively improved using a phase shift mask.
Further, since the application of the optical proximity correction mask requires fine corrections to the photomask pattern, the design and the fabrication of the photomask becomes tedious. When the pitch of the pattern is too small, there is no space for forming the assistant features on the photomask. As a result, the photoresist layer pattern may suffer from corner rounding. The resulting opening in the wafer layer is likely formed with a rounded profile. The exact shape, cross sectional area and resistance of the contact hole/via hole are thus difficult to control.
A method of forming an opening in a wafer layer is provided. At least two patterned photoresist layers are formed on a wafer layer. Each photoresist layer comprises patterns of various configurations, wherein spaces are present between the patterns in each photoresist layer. The photoresist layers are stacked substantially over each other to form an opening pattern that exposes the underlying wafer layer by superpositioning the spaces between the patterns of two photoresist layers. The wafer layer exposed by the opening pattern is then etched to form an opening, for example, contact/via holes or trenches. Since there are no corner in the line/space type of pattern, the opening pattern resulted from the superpositioning of spaces between patterns of the photoresist layers can avoid corner rounding and the proximity effect. The profile and the cross-sectional area of the opening formed on the wafer layer in a subsequent etching process are thus easier to control.
Both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.